1. Field of the Invention
This invention relates in general to an impact device for delivering blows to a working tool, and in particular to an improved hydraulic cocking mechanism for an impact tool.
2. Description of the Prior Art
Many types of impact tools for mining and breaking up concrete and the like are known. These devices have essentially an energy storage means such as a coil spring or gas chamber, a hammer, and a working tool. The energy storage means when compressed, causes the hammer to accelerate to the working tool to deliver a blow.
Various mechanisms are used to automatically recock the hammer to repeat the cycle and to release the hammer for the blow. Some are mechanical, using a rotating cam and cam follower. Others are hydraulic. The hydraulic types usually have a piston attached to the hammer for urging it upward to compress the energy storage device. Some have external control valves to alternately supply fluid to the piston chamber to cycle the hammer. Others have internal valves for automatically cycling the fluid to the piston chamber.
Some of the internal valve types have a solid piston, such as shown in U.S. Pat. No. 2,559,478. However, this requires that the piston push the chamber fluid out an exhaust port on the downstoke. Appreciable drag may result. Others of the internal valve type avoid this by having a piston made up of two separate components. For example, in U.S. Pat. No. 3,687,008, the shaft has an annular stop fixed to it that is smaller in diameter than the compression chamber. On the upstroke, the annular stop contacts a sleeve, which combines with the annular stop to define a piston. At the top of the stroke, the annular stop and sleeve separate with the annular stop and shaft going to impact. Since the annular stop is smaller in diameter than the chamber, it does not have to push all of the fluid below it out an exhaust port on the downstroke. At the bottom of the stroke, the sleeve is reseated with the annular stop. On the upstroke, a resistance or bias has to be applied to the sleeve, otherwise pressure below it would push it off of its seat with the annular stop. Also, some pressure from above has to be applied after the downstroke begins in order to push the sleeve back into seating contact with the annular stop. The device of U.S. Pat. No. 3,687,008 accomplishes this by placing a port at the top of the compression chamber with a fairly large back pressure.
U.S. Pat. No. 3,866,690 avoids having to bias the sliding sleeve into contact with the shaft annular stop by placing the sleeve below the annular stop. On the upstroke, the sleeve pushes the annular stop upward. At the top of the stroke, the sleeve must be returned to starting position first, then the annular stop is forced to impact by the gas spring.
U.S. Pat. No. 3,792,738 avoids having to push the piston chamber fluid out on the downstroke by using a piston with a central passage through it. A cylindrical valve above it is biased into contact with the piston passage to seal the passage on the upstroke. At the top of the stroke, the valve and piston separate opening the passage. This reduces the hydraulic pressure below the piston to allow the spring to push it downward. On the downstroke, fluid in the piston chamber passes through the central passage. The valve is biased and returned by the same hydraulic input pressures that drive the piston upward. The valve's pressure area is smaller than the piston, however, to provide a net upward force for cocking.
While these proposals may be suitable, improvements to an internally-valved hydraulic cocking mechanism are desired. In particular, means to absorb sealing surface shock as the annular stop and sleeve reseat is desirable.